1. Field of the Invention
Aspects of the present invention relate to an electrolyte for a lithium ion secondary battery and a lithium ion secondary battery comprising the same. More particularly, aspects of the present invention relate to an electrolyte for a lithium ion secondary that can prevent a thickness increase of a lithium ion secondary battery and thus provide improved thermal stability and durability of the battery even when the battery is placed at a high temperature, by containing a phosphonitrile fluoride trimer.
2. Description of the Related Art
A battery is a device that converts the chemical energy of chemical materials inside the battery into electrical energy through an electrochemical oxidation/reduction reaction. Recently, portable devices such as camcorders, cellular phones, notebook computers, PCs and PDAs have been actively developed with rapid progress of the electronic, telecommunication and computer industries. Accordingly, there has been an increased demand for a slim secondary battery of high performance, durability and reliability that can be used in the above portable devices.
Lithium metal has a low weight and standard electrode potential. Accordingly, lithium has very high operation voltage and energy density when it is used as an anode of the battery. However, lithium metal can cause instability of the battery by reacting with an organic solvent to cause an electric short in the battery. Thus, carbon has been developed as an anode material that can substitute for lithium metal. Carbon materials are capable of reversible intercalation and deintercalation of lithium ions because such materials have an electrode potential most similar to the lithium metal and a layered structure. Such a battery is called as a lithium ion battery. The lithium ion battery has been widely used as a secondary battery because it has a high discharge voltage near 4V, excellent energy density per weight and low self-discharge rate.
During initial charging of the lithium battery, lithium ions released from lithium metal composite oxide used as a cathode move to a graphite electrode used as an anode and are inserted into layers of the graphite electrode. At that time, the lithium ions form a kind of a passivation layer on the surface of the graphite anode. The passivation layer is called as an SEI (solid electrolyte interface) film. When the SEI film is formed, the SEI film functions as an ion tunnel to allow only lithium ions to pass through. By the effect of the ion tunnel, molecules of organic solvent having a large molecular weight, which move with the lithium ions in the electrolyte, such as, for example, EC, DMC or DEC are inserted into the graphite anode to prevent degradation of the graphite anode structure. Once the SEI film has been formed, the lithium ions do not react with the graphite anode or other material again. The amount of the lithium ions in the electrolyte is kept reversible and thus stable charge/discharge characteristics are maintained. However, in an angular thin film battery, gases such as CO, CO2, CH4, C2H6 are generated by decomposition of a carbonate group organic solvent during the above SEI formation reaction to cause a problem that the thickness of battery expands during charge. In addition, when the fully charged battery is stored at a high temperature, the SEI film is slowly degraded over time. Accordingly, a sub-reaction, that is, a reaction between the exposed anode surface and the electrolyte around it is continuously generated and the internal pressure of the battery is increased by the continuous generation of gas. As a result, in the case of the angular battery, the thickness of the battery increases. Accordingly, there has been increased desire for an electrolyte that has excellent thermal properties without expansion of the volume when the battery is stored at a high temperature.
In addition, most non-aqueous electrolytic solvents typically have a low withstand voltage. When electrolytes containing solvents having a low withstand voltage are used in the secondary battery, the solvent decomposes upon repetition of charge/discharge cycles and thus gas is generated to cause the increase of the internal pressure of the battery. As a result, the charge/discharge efficiency of the battery is decreased and the lifetime of the battery is shortened by reduction of the energy density of the battery.
To solve the above problems, there have been proposed batteries that improve the thermal property by adding a small amount of certain compounds as an additive in the electrolyte of the lithium secondary battery (See Japanese Patent publications No. 1996-22839 and No. 1990-10666).